Electric discharge tube having an oxide cathode



Patented July 25, 1939 PATENT OFFICE ELECTRIC DISCHARGE TUBE HAVING AN OXIDE CATHODE Wilhelm Gerard. Burgers, Hajo Bruining, and

Petrus Hubertus Johannes Agnes Kleynen,

Eindhoven, Netherlands, assignors to N. V.

Philips Gloeilampenfabrieken,

Netherlands Eindhoven,

No Drawing. Application March 9, 1937, Serial 2 Claims.

This invention relates to electric discharge tubes having an oxide cathode.

With the use of electric discharge tubes comprising a cathode and an anode together with 6 one or more grids therebetween it has for a long time been attempted to avoid the drawback that the electrons issuing from the cathode dislodge secondary electrons upon striking at a high speed one of the remaining electrodes, which may give 10 rise to undesirable phenomena in the tube. Various means have already been resorted to for obviating this secondary emission. It has been proposed, for instance, to coat grids or anodes of discharge tubes with materials such as chromium oxide, nickel oxide and the like. Furthermore the use of black material such as zirconium in the black form has been suggested, while it is also known to make electrodes from zirconium or to coat them with zirconium oxide.

Although various of these materials entail indeed a decrease in secondary emission the drawback very often manifests itself that when electrodes whose surface is coated with such mate- 26 rials are used in combination with an oxide cathode, the secondary emission of these surfaces increases during operation. Probably this is due to very slight quantities of barium vaporizing from the cathode and depositing on the surface of the other electrodes where they form a source of secondary electrons.

We have now found upon making various experiments that these defects can be avoided by using a discharge tube according to the invention. Such a tube comprises an oxide cathode and in addition one or more other electrodes one or more of said other electrodes being coated at at least part of their surface with a layer of soot directly obtained from the gaseous phase. Those parts of the surface of this electrode are preferably coated with such a layer, which can be struck by any barium vaporizing from the cathode. Of course, the portion facing the cath- 46 ode enters more particularly into consideration for this purpose. Such a soot layer is provided In the Netherlands March 24,

with great advantage by holding the surface of this electrode above burning turpentine.

Although we are not quite sure how to explain the effect obtained by the invention it is not excluded that due to the structure of such soot layer the barium vaporizing from the cathode is held in such a manner, or may perhaps penetrate into the carbon layer in such a manner that no secondary electrons can be dislodged therefrom. In fact, it appears that smoother carbon layers exhibit a much smaller effect in this respect.

Carbon layers according to the invention give a good electron-diffraction image, whereas layers which have not directly been obtained from the gaseous phases probably have a much smoother surface structure, which appears from the impossibility of obtaining a good electron-diffraction image.

The invention will be more clearly understood by reference to one form of example.

A number of anodes in an electric discharge tube is held in the flame of burning turpentine and is thus covered with a soot layer. After that these electrodes can be brought together in the usual manner with grids and cathode in a discharge tube, the latter being finished in a known manner.

What we claim is:

1. In an electron discharge tube provided with an oxide coated cathode and one or more metalhe electrodes cooperating therewith, the method 

